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Investigation of the influence of severe plastic deformation on the microstructure and mechanical properties of Al-7136 alloy

INVESTIGATION OF THE INFLUENCE OF SEVERE PLASTIC DEFORMATION
ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF
Al-7136 ALLOY
by
Zhichao Duan
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(MECHANICAL ENGINEERING)
August 2009
Copyright 2009 Zhichao Duan

The super-saturated Al-Zn-Mg-Cu alloys or 7000 series Al alloys are increasingly replacing the 2000 series Al alloys in aerospace applications due to its outstanding strength. The potential of further strengthening the 7000 series Al alloys using SPD methods makes it tempting to conduct ECAP as well as HPT on the selected Al-7136 alloy.; ECAP processing at 473 K was proven to be feasible for Al-7136 alloys as there was no cracking or segmentations of the rods after 8 passes of ECAP. TEM study revealed that this could effectively refine the grain size and achieve an equiaxed microstructure in Al-7136 after 8 passes of ECAP. However, the mechanical properties of the as-pressed Al-7136 were mostly deteriorated compared to the as-received material.; To avoid the negative effect of processing at 473 K, an innovative ECAP processing routine including solid solution, quenching and immediate room temperature ECAP pressing was successfully carried out on Al-7136 rods. The room temperature mechanical properties were consequently strengthened but still it failed to improve the high temperature performance.; 3-D Atom Probe Tomography (3-D APT) as well as X-ray diffraction (XRD) revealed a promotion of the precipitates growth and the segregation of the alloying elements at the grain boundaries in the as-pressed samples, they are considered as the mains reasons for the softening of the Al-7136 alloy despite of the grain refinement.; HPT was also conducted on Al-7136, besides direct HPT processing, different pre-processing of ECAP including 1 pass room temperature pressing and 4 passes pressing at 473 K were also employed before HPT processing, respectively. A complicated microhardness evolution dependent on the pre-processing as well as strains imposed was found.; The effect of HPT on the grain refinements and diffusion coefficients of alloying elements were discussed based on the TEM images of grain size and precipitates. It was concluded that HPT processing could significantly refine the grain size, meanwhile, the diffusion coefficients were much higher in materials after HPT processing that would possible lead to similar precipitation and grain boundary segregation as that in ECAP. This could successfully explain the hardness evolution mentioned earlier.

INVESTIGATION OF THE INFLUENCE OF SEVERE PLASTIC DEFORMATION
ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF
Al-7136 ALLOY
by
Zhichao Duan
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(MECHANICAL ENGINEERING)
August 2009
Copyright 2009 Zhichao Duan